The company, controlled by billionaire Xavier Niel, reported like-for-like revenue growth of almost 5% for the first three months of the year, helped by a rise in mobile and broadband subscribers.
But Credit Suisse and JP Morgan analysts said the figures for its main French market and for Italy were slightly weaker than expected, while Poland was ahead of forecasts.
Iliad shares fell as much as 8% to a one-year low of 139.4 euros. British mobile operator Vodafone also missed market expectations for full-year earnings on Tuesday, after the pandemic hit roaming revenue and handset sales.
Iliad said it would review its 2021 cash flow target for France in order to speed up spending on 5G networks in the country, where it launched the cheapest offer of the four main operators late last year.
To help ramp up spending, the Paris-based group said it would sell its 30% stake in On Tower France, which it values at a minimum of 600 million euros ($731 million).
Iliad, which had previously guided for a French operating free cash flow of around 900 million euros this year, said it would give a new target in September, also taking into account a global shortage in semiconductor components.
The company said it expected to turn a profit from its Italian business this quarter - sooner than previously forecast - but delayed the launch of its broadband offer there until after the summer due to the pandemic.
In Poland, it said its integration of mobile operator Play - bought late last year - was proceeding ahead of schedule, with 8,000 new customers added over the first three months of 2021. $1 = 0.8208 euros Reporting by Sarah Morland and Boleslaw Lasocki in Gdansk. Editing by Mark Potter