It is not uncommon to form friendships with co-workers, however, your husband seems to have the intentions of taking the friendship status to another level.
You have a legitimate reason to have concern about your husband being in contact with another woman, but it is important to not give this more power over your life than it deserves. It is natural for you to be curious about the other woman, but do not obsess over her. Do not pick yourself apart comparing yourself to her and contemplate reasons why your husband is in contact with her. You need to confront your husband, but first make an effort to be strong and not concern yourself with the unnecessary painful thoughts that will in no way make you feel better or get your marriage back on track.
Confront your husband in a non-aggressive manner with the facts only.
There are many outcomes when a confrontation is initiated. Your husband will either admit to having contact with the other woman, deny that it even happened, get on the defensive and make excuses, or try to change the subject. It is essential that you know beforehand what you want out of the conversation and keep your husband on the topic at hand. If you feel that you are not getting what you want out of the conversation, or if things start to escalate, take a break from the conversation and continue again when you feel ready.
Good communication will be the key to your road to recovery, so be sure to ask the right questions and listen with undivided attention and understanding.
After admitting to having contact with the other woman, your husband needs to examine the personal reasons that caused the "phone relationship".
The text message is important because it gives a little insight about why he is staying in contact with the other woman. Ask your husband what it is about the other woman that makes her his "ideal woman", what needs is she meeting (if any) that you are not?
Know that this has nothing to do with your husband's love for you, nor does it make you a failure in the marriage. What this does tell you though, is that there are essential issues that need to be addressed.
Determine why your husband is looking outside the marriage. Talk to your husband about the situation and the reasons that drove him to it. The focus is to find and establish the reasons for his calling/texting the other woman (was it for emotional support, etc.) and ways you can prevent the same event in the future so that you can move on with your lives together.
It is normal to expect some boundaries and rules when it comes to the opposite sex, address those concerns NOW. All contact with the other woman (outside of normal working situations) must stop, any emotional support that he needs should be met by you. In time he will realize that and will no longer need the "phone relationship" with the other woman.