Thanks for all the replies.
The situation is as follows (I can't give too much away). My sister (real, fleshly sister - sorry - I hate that expression too, but just clarifying) is married to a pioneer who has been doing the texting/phoning. We suspect the person he is texting (an elder's wife) has also been doing the same - texting/phoning many times a day for at least the last 6 months. My sister happened to check her husband's mobile phone bill and saw dozens of calls every day from him to her, and she realised that he has also been getting more texts and calls than usual.
The funny thing is, he has made calls to this woman at times when they were both in the house but she has NEVER heard him phoning her - so it is obviously being done in secret - in the garden, upstairs etc. She has never heard him having a conversation with her by phone - how odd! She checked some of his old phone bills and this has been going on for at least 6 months (as far back as she can find bills for). The elder's wife is also a pioneer and she is looked up to by most of the congregation.
My sister confronted her husband and he swears it is all to do with the ministry, calls, Bible studies etc. He says she is being ridiculous and it is entirely innocent. My sister is in bits - she had a feeling things weren't right - he has been pretty distant lately and goes out for long periods. Personally, I can't stand my brother-in-law - he is a total prat! But I don't really know what to say to my sister - I don't know how to advise her. I am not in the same congregation as my sister - she is up north - but we speak regularly by phone. I hardly go to meetings any more - I can't hack the hypocrisy and lies.
From your replies so far, and my own gut feelings, I definitely think there is more to this than 'ministry-related' stuff!