The two companies have ownership ties overseas, with T-Mobile USA majority owned by Deutsche Telekom and Verizon Wireless partly owned by UK-based Vodafone. It is thought legal complications could arise if they are involved in the programme.
The National Security Agency is stepping up efforts to monitor internet and phone data of millions of Americans.
The chances of the two companies not actually being monitored are small as calls and data will pass through networks controlled by US companies at some point that does work with the NSA, said a US official.
AT&T and Sprint have to disclose information on calls over their networks and are thought to participate actively in the programme.
Companies including Google, Microsoft and Facebook have come under scrutiny in US national press regarding their role over the issue in the last week.
