T-Mobile and Orange have offered to relinquish up to a quarter of the frequencies they hold in the 1,800MHz band to speed up a favourable decision from regulators looking at the proposed merger of their UK businesses.
Other mobile network operators (MNOs) believe this is too little, the Financial Times reported.
The merger would create the UK's biggest MNO with some 29.5 million subscribers, leaving O2, Vodafone and 3 to share the rest of the 77 million market.
The spectrum the two have offered is needed to provide high speed broadband in cities. But the offer, made to the European Commission, may still leave the merged company with the biggest share of 1800MHz spectrum in the UK.
O2, the current market leader, 3 and consumer groups have asked UK competition authorities to review the proposed merger.