Proxy advisers Glass Lewis and ISS have backed efforts by two key activist investors to block the deal.
MetroPCS said in a letter to shareholders on Monday there could be no assurance that it would be able to deliver better shareholder value as a stand-alone wireless company.
The company will hold a special meeting of stockholders on April 12 to vote on the deal.
Glass Lewis said on Friday that the deal undervalued MetroPCS's contribution to the combined company. ISS said MetroPCS could thrive as a stand-alone company.
Paulson & Co, the biggest MetroPCS shareholder, and P. Schoenfeld Asset Management, another big shareholder, have said they will vote against the deal.
Madison Dearborn Partners, the second-largest shareholder, has thrown its weight behind the deal.
Under the terms of the reverse merger announced in October, Deutsche Telekom would end up with 74 percent of the combined company, and MetroPCS would declare a 1-for-2 reverse stock split and pay $1.5 billion in cash to its shareholders.
T-Mobile USA, the No. 4 mobile provider in the U.S., and MetroPCS want to pool their spectrum resources and networks in order to better compete with larger rivals Verizon Wireless, AT&T, and Sprint Nextel.