MetroPCS Communications Inc. has called shareholders for support for its planned merger with Deutsche Telekom AG's unit T-Mobile USA Inc. after two proxy advisory firms suggested that investors should veto the deal.

Proxy advisers ISS and Glass Lewis have recently recommended that shareholders should block the merger as they back the efforts of two key activist investors.

In a letter addressed to shareholders, MetroPCS said that allowing it to stand as sole business provides no assurance that it would be able to deliver shareholder value.

Under the terms of the reverse merger declared in October of last year, T-Mobile's parent company would have 74% of the combined firm while MetroPCS would pay $1.5 billion to its shareholders once it declares a 1-for-2 reverse stock split.

A special meeting will be held by MetroPCS shareholders on April 12 to vote on the merger.