It is entirely possible that they are just friends, though the fact that this is bothering you is something that makes it important. Have you talked to him about this? If not maybe now is the time to let him know how these calls make you feel.......even if she is just a friend he must respect your feelings.
Though you all so must respect the fact that he has female friends and as such not suspect him every time they speak. This can be hard to do if you suspect they are having a affair.
You may want to ask him how he would feel if the shoe was on the other foot and you had a male friend such as this. What makes you think he is texting but erasing the messages? If you know for a fact he is or you can prove this then it may be time to sit down and ask him why he continues to text her and then erase it. Erasing text messages makes him look guilty. So he is either doing this because he is cheating, or doing it because he is innocent and does not want you to think he is doing something he should not be doing.
Either way, dishonesty tends to breed distrust. He can not be dishonest with you, it is not fair to you regardless of if it is friendly conversations or not. Have you spoken with the women? If not maybe a friendly conversation is in order......this is not to say you should accuse her. But simply let her know that you know about the friendship and want to know if it is more. Women to women. Most women will be honest....if they are out to take your man they tend to let the other women know. Though a word of caution....do not go to her accusing her or angry. Just go to her as a concerned wife, let her know you are not accusing her but you are worried and wanted to know if there was anything to be worried about.
I would then suggest letting your husband know this is not appropriate for him to be texting her and hiding the fact that he is doing so. Let him know how you feel and what you need. If this is a breaking point for you, then you need to be honest with him. Do not give him a ultimatum but if you are planning on leaving over it then he has a right to know so he can fix this if he is innocent.
The only one who knows if he is cheating or not is him and her. You can either choose to believe him or choose not to. But if you plan on sticking around then it is time to confront him and let him know this is not something you will tolerate if he is hiding it.