Solicitors have been alerted to the risk of "inadvertently" committing a criminal offence by calling clients in prison who are using illegal mobile phones.
The Law Society issued a note to its members warning they could be taking part in criminal activity by ringing the prohibited phones.
According to the guidance, some lawyers may call inmates "without realising the call is being made to an illicitly possessed telephone".
The note reads: "There is a danger that by making a call to a prisoner's unlawfully possessed mobile telephone you may be, either deliberately or inadvertently, committing a criminal offence."
The Society said the illicit possession and use of mobile telephones in prisons is becoming an increasing issue and warned solicitors not to call inmates on mobile phone numbers and to terminate any incoming calls from prisoners using mobiles.
It said: "You should inform the caller that they are committing an offence and that you and your staff will not accept calls in such circumstances."
Illegal mobile phones in prisons have been used to enable drug deals and even organise a murder.
A gang member in Liverpool was killed in a hit organised by an inmate using a illicit mobile phone.
Liam Smith was shot dead outside Merseyside's Altcourse Prison on August 23 2006. He was spotted visiting a friend in the prison by rival gang member, inmate Ryan Lloyd, who used a hidden mobile phone to arrange the hit.
Mark Leech, editor of Converse, the national prisoner's newspaper, said mobiles in prisons are worth £2,500 to £5,000 each month to convicts, who charge other inmates to use them.