Residents in parts of mid Wales have raised concerns about their reception.
One said it can even take up to five hours to receive a text message.
Montgomeryshire AM Russell George, chairman of the cross-party group on digital communications, has called for a meeting of mobile operators in the area.
Wayne Evans, of Newtown, in Powys, claimed his mobile service has become increasingly unreliable in the past few months.
"It's very frustrating," he said.
"It takes four to five hours before a text message will come through. It takes an hour to get through to Twitter on the internet. It's just appalling really."
Mr George said he has been contacted by frustrated residents in his constituency about the issue.
He called for a "concerted effort" by mobile operators to tackle signal problems in the region.
"In recent years, we have heard a lot about the launch of new 4G services and new network-sharing agreements between the operators, which are supposed to have improved coverage and speeds, but the reality in some rural areas is that mobile coverage has actually got worse," he said.
"I believe that mandatory roaming and compulsory sharing of mobile infrastructure are the two most practical solutions which will resolve this issue."
He hopes to meet with mobile operators in November.
Ofcom's regulatory affairs manager for Wales, Elinor Williams, said the problem is due to a programme of "mast rationalisation".
Mobile phone operators have agreed to share masts and are switching off some to find out which have the strongest signals.
"They are working out which ones are the most powerful, which services the greatest population and, therefore, which ones are the most profitable to them," she said.
"It's a programme of hit and miss at the moment, I think. In time it will settle down."
Mobile phone users in rural areas have complained that their service is getting worse, despite 4G roll-out and network-sharing agreements.